In blood samples, many substances are present in the form of a complex of two different substances. For such kinds of complexes, transferrin-transferrin receptor complex, thrombin-antithrombin complex, factor VIIa-antithrombin complex, oxidized LDL-CRP complex, PSA-α2-macroglobulin complex, PSA-antichymotrypsin complex, oxidized LDL-α1-antitrypsin complex, protein C inhibitor-protease complex, plasmin-α2-plasmin inhibitor complex, PSA-protein C inhibitor complex, plasminogen activator inhibitor-tissue plasminogen activator complex, haptoglobin-hemoglobin complex, transthyretin-retinol binding protein complex and the like are known.
As methods for identifying these complexes like a complex of substance A and substance B, or for assaying the levels thereof, the following examples are known. The first one is a method using an electrophoresis (Patent Literature 1). The second one is a method where the complex in a sample is determined by allowing the complex to get contact with a solid phase antibody made through binding either one of an antibody against substance A and an antibody against substance B to a solid phase, and simultaneously to react with a labeled antibody derived from the other antibody that is not immobilized to the solid phase (Patent Literature 2). The third one is a method of assaying the complex by various kinds of immunoassay using an antibody specific to the complex but with low affinity for the individual released components of the complex (Patent Literature 3). The first method, however, has a disadvantage that the sensitivity and quantitativeness are low and that the operation is complicated. The second method has a problem that a general-purpose automatic biochemical analyzer cannot be applied for the assay owing to the utilization of a solid phase, requiring much time in washing procedure, reaction and so on. In the third method, there has been a problem that it is difficult in some cases to obtain an antigen to be used for the production of an antibody against the complex or to obtain a specific antibody, and that it is also difficult to assay the complex accurately owing to a matter of specificity. Thus, in a homogeneous system, there is a need for the development of a simple method for assaying complexes, for which necessary reagents are readily available and which is applicable to automatic biochemical analyzers.
Meanwhile, it has recently been reported that an IgA-albumin complex was isolated from five patients with IgA-type M-proteinemia and that the complex was confirmed to be present by a Western Blot (Non Patent Literature 1), whereas no simple method of measuring its level is known.